Field of the Invention
The present invention relates to a power supply device including a boosting circuit.
Description of the Related Art
A portable device such as a digital camera uses a liquid crystal display device as a display device for displaying an image or the like. A liquid crystal display device generally uses as a backlight circuit a lighting circuit including white light emitting diodes (LEDs). A liquid crystal display device having an approximately 3-inch display unit often uses a lighting circuit in which four to six white LEDs are connected in series. Since a forward voltage Vf of each white LED is about 3.0 V, a power supply of about 12 to 18 V is required to drive the lighting circuit. Recently, mainstream digital cameras employ a system using a one cell lithium-ion battery or two cell lithium-ion battery. In such a system, a voltage input from the battery is 3 to 9 V. Thus, a boosting circuit is required to drive the above lighting circuit.
Further, to remove dust attached to an image sensor when a lens is replaced in an interchangeable lens digital camera, there has been proposed a technique for vibrating a low pass filter (a quartz plate) placed in the image sensor using a piezoelectric element, thereby removing the dust. In this case, it is necessary to drive the piezoelectric element at a relatively high voltage, namely about 20 to 25 V. Thus, an interchangeable lens digital camera using a one cell lithium-ion battery or two cell lithium-ion battery requires at least two types of boosting circuits. A first boosting circuit is a boosting circuit used for a backlight circuit of a liquid crystal display device, and a second boosting circuit is a boosting circuit used to drive a piezoelectric element. These boosting circuits need to employ a high voltage semiconductor device having excellent switching responsiveness and therefore are relatively expensive.
Japanese Patent Application Laid-Open No. 2006-101637 describes a technique for generating, in a single boosting circuit, driving voltages for driving a first load circuit and a second load circuit, which is driven at a higher voltage than the first load circuit.
A system described in Japanese Patent Application Laid-Open No. 2006-101637 includes two types of feedback circuits (30, 40) for two load circuits (80, 82). A first feedback circuit (30) is a circuit that is highly efficient as a circuit for passing a constant current through LEDs (80), which are a first load circuit. A second feedback circuit (40) is a circuit used to drive a second load circuit (82). If the second feedback circuit (40) is used, excessive power is supplied to the LEDs (80). Thus, a constant-current circuit (42) is connected to a cathode side of the LEDs (80). When the second feedback circuit (40) is used, however, a feedback voltage is varied depending on individual variations and temperature variations in a forward voltage Vf of the LEDs (80). This results in varying an output voltage from a boosting circuit (10).
Further, in the system described in Japanese Patent Application Laid-Open No. 2006-101637, the first feedback circuit (30) may not be able to perform a feedback control for generating a constant current under a condition that an input voltage of the boosting circuit (10) is higher than an output voltage of the boosting circuit (10). For example, the first feedback circuit (30) may not be able to perform the feedback control under a condition that an input voltage from a power source is 5 V to 10 V and a driving voltage of the LEDs (80) operated as a backlight circuit is about 9 V. In such a case, the first feedback circuit (30) cannot perform a constant-current control, and therefore, it is necessary to perform the constant-current control using only the second feedback circuit (40). The system described in Japanese Patent Application Laid-Open No. 2006-101637, however, does not include a circuit for determining such condition. Japanese Patent Application Laid-Open No. 2006-101637 does not describe a condition for determining such condition, either. Further, in the system described in Japanese Patent Application Laid-Open No. 2006-101637, when the first feedback circuit (30) is in a connected state and if the lighting up of the LEDs (80), which operate as the backlight circuit of a liquid crystal display device, has been started, an illuminance of the liquid crystal display device gradually increases. Thus, the liquid crystal display device may display an uncomfortable image.